[XTarget] Disallow Corpses in XTarget Auto Slots (#1881)

* [XTarget] Disallow Corpses in XTarget Auto Slots

Why:
There exists an odd state where corpses will fill up your XTarget window.
This is reproducable using a combination of a pet to kill a mob
and timely feign death to wipe the owner's aggro.

What:
Added an IsCorpse check to IsXTarget.
Added a block to mark corpse XTargets as dirty to ProcessXTargetAutoHaters

* fixup! [XTarget] Disallow Corpses in XTarget Auto Slots

* fixup! [XTarget] Disallow Corpses in XTarget Auto Slots

* [XTarget] Disallow Corpses Code Cleanup

Added some safety, performance, and code readability changes per PR request.

zone/mob.h

@@ -1307,6 +1307,8 @@ public:
 	std::string GetBucketRemaining(std::string bucket_name);
 	void SetBucket(std::string bucket_name, std::string bucket_value, std::string expiration = "");
 
+	bool IsValidXTarget() const;
+
 #ifdef BOTS
 	// Bots HealRotation methods
 	bool IsHealRotationTarget() { return (m_target_of_heal_rotation.use_count() && m_target_of_heal_rotation.get()); }